Magnetic tunneling transistor (MTT) devices have been developed to improve the performance of magnetic sensors. MTT devices evolved from earlier spin valve transistor (SVT) devices.
A SVT device is a three terminal device that is described using the emitter, base and collector nomenclature of the bipolar junction transistor (BJT). However, unlike a BJT, in which all three layers are made of semiconductors, a SVT typically consists of a semiconductor emitter, a multi-layered metallic base, and a semiconductor collector. Because the emitter and collector are made of a semiconducting material, while the base is made of a metallic material, Schottky barriers are formed at the emitter-base junction and the base-collector junction. These Schottky barriers prevent electrons at the Fermi level from traveling through the structure. Materials are selected so that the Schottky barrier between the emitter and the base is higher in energy than the Schottky barrier between the base and the collector. A typical emitter-base Schottky barrier height is 1 to 2 electron-volts (eV), while a typical base-collector Schottky barrier height is 0.5 eV.
In the operation of a SVT device, a current is established between the emitter and base, such that electrons are injected into the base perpendicular to the layers of the SVT. Since the electrons must overcome the emitter-base Schottky barrier, the electrons enter the base as non-equilibrium or “hot” electrons. The energy of these hot electrons is determined by the height of the Schottky barrier between the emitter and the base.
The base region of a SVT filters the hot electrons according to their spin. The base region usually has three layers: a fixed ferromagnetic layer with a fixed magnetic orientation, a free ferromagnetic layer with a magnetic orientation that is free to rotate in the presence of a magnetic field, and a non-magnetic metal layer separating the fixed and free ferromagnetic layers. The fixed ferromagnetic layer filters electrons according to their spin. It allows electrons with one spin orientation (up or down) to pass through it, but prohibits transport of electrons with the opposite spin. The free ferromagnetic layer will change its orientation in the presence of a magnetic field. It will allow electrons with one spin orientation to pass through it, while prohibiting electrons with the other spin orientation from passing through it. If the free ferromagnetic layer has the same magnetic orientation as the fixed ferromagnetic layer, both ferromagnetic layers will allow electrons with a certain spin orientation to pass through. Therefore, electrons will be able to flow through the base to the Schottky barrier between the base and the collector. However, if the free ferromagnetic layer has the opposite magnetic orientation of the fixed ferromagnetic layer, the two layers will each filter electrons with different spins. As a result, electrons will not be able to traverse the base.
Thus, the presence of a magnetic field results in the SVT having one of two states. If the magnetic field that is present has one orientation, current will flow through the base. If the magnetic field that is present has the opposite orientation, current will not flow through the base.
As the hot electrons traverse the base, they are subject to scattering, which changes both their energy and momentum distributions. A hot electron is only able to enter the collector if its momentum matches one of the available states in the collector. In addition, hot electrons are only able to enter the collector if they have retained sufficient energy to overcome the base-collector Schottky barrier, which is chosen to be somewhat lower than the emitter-base Schottky barrier. This creates a significant limitation on the device. Because the amount of electron scattering increases with the distance the electrons travel, in order for the hot electrons to retain sufficient energy to pass over the second Schottky barrier, the base region must be thin. Since the base is composed of multiple layers of metal, this presents practical problems.
A voltage between the base and collector does not affect the hot electron current of the SVT because a voltage does not significantly change the maximum of the Schottky barrier when measured with respect to the Fermi energy. Similarly, a change in the emitter-base voltage does not affect the energy at which the hot electrons are injected into the base, because the applied voltage hardly modifies the maximum of the emitter-base energy barrier. As a result, the collector current in a SVT device is linearly proportional to the emitter current.
The MTT evolved from the SVT and is sometimes referred to as the tunnel SVT. The MTT differs in structure from the SVT in that the emitter of the MTT is made of metal. Therefore, no Schottky barrier is formed between the emitter and the base. In place of the Schottky barrier, a tunneling barrier prevents low energy electrons from traveling from the emitter to the base. The base and the collector of the MTT have the same structure as the SVT, so a Schottky barrier exists between the base and the collector. Like SVT devices, MTT devices are designed so that the tunneling barrier is higher in energy than the Schottky barrier at the base-collector junction.
The MTT, like the SVT, is a hot carrier device. However, because the emitter-base junction of the MTT has a tunneling barrier instead of a Schottky barrier, one notable difference between the MTT and the SVT is that the energy of the barrier between the emitter and the base is much more strongly affected than a Schottky barrier by the voltage applied between the emitter and the base. This means that the energy of the hot electrons that are able to pass the tunneling barrier may be adjusted over a certain range of energies. Raising the hot electron energy increases the ratio of the collector current over the emitter current. This results in the MTT having a larger collector current than a comparable SVT.
While the MTT demonstrates improved performance over the SVT, most notably in the larger collector current of the MTT, its performance is still insufficient for many uses. The collector-emitter current ratio, or transfer ratio, of MTT devices is typically of the order 10−3, which is insufficient magnetocurrent for industrial applications. In addition, the presence of a collector made of a single-crystal semiconductor material prevents the practical use of MTT devices for many applications, such as recording heads.
Therefore, to improve the performance of MTT devices and utilize them in applications such as magnetic sensors in a recording head, it is desirable to create MTTs with larger collector-emitter current ratios. It is also desirable to create MTTs that do not require single-crystal semiconductors.